The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 114, Issue 6
Displaying 1-6 of 6 articles from this issue
Articles
  • Jun-ichi Tazawa, Satoshi Hasegawa, Yoshiyuki Hasegawa
    2008 Volume 114 Issue 6 Pages 269-285
    Published: June 15, 2008
    Released on J-STAGE: March 22, 2009
    JOURNAL FREE ACCESS
    The Upper Permian (Lopingian) Mizukoshi Formation, distributed throughout the Mizukoshi area of central Kyushu, southwest Japan, is composed chiefly of black shale along with intercalated sandstone and conglomerate (Usuginu-type conglomerate), making a total thickness of 1,690 m. The Mizukoshi Formation is lithologically similar to the Toyoma Formation in the South Kitakami Belt, the upper Moribu Formation in the Hida Gaien Belt, and the Kuma Formation in the Kurosegawa Belt. The fusulinoidean fauna from limestone clasts within conglomerates in the upper part of the Mizukoshi Formation is characterized by the presence of Lepidolina species and the absence of Neoschwagerina and Yabeina species. In terms of generic composition, the fusulinoidean fauna of the Mizukoshi Formation shows a close affinity to those of the South Kitakami and Hida Gaien Belts, but differs from that of the Kurosegawa Belt. The brachiopod fauna from sandy shale and calcareous sandstone in the upper part of the Mizukoshi Formation, consisting of 22 species in 19 genera, is a Boreal- Tethyan mixed fauna, although the Boreal types are predominant. The Mizukoshi fauna is allied with the Middle to Late Permian brachiopod faunas of the Hida Gaien and South Kitakami Belts, the fauna of South Primorye, eastern Russia, and especially with the fauna of the Moribu area in the Hida Gaien Belt, central Japan. The above data strongly suggest that the Mizukoshi Formation represents the SW extension of the Permian rocks of the Hida Gaien Belt, thereby placing the formation within the South Kitakami Terrane. The dominantly Boreal-type assemblage within the Mizukoshi fauna indicates that during the Middle- Late Permian the Mizukoshi area was probably located between the Hida Gaien Belt to the north and the South Kitakami Belt to the south, bordering the eastern margin of North China. This conclusion supports the strike-slip model that describes large-scale sinistral strike-slip movement (approximately 1,500-2,000 km of displacement) along the Tanakura Tectonic Line (TTL) -Median Tectonic Line (MTL) from the Early Cretaceous to the Palaeogene.
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  • Eiichi Ishii, Ken-ichi Yasue, Hiroto Ohira, Akira Furusawa, Takeshi Ha ...
    2008 Volume 114 Issue 6 Pages 286-299
    Published: June 15, 2008
    Released on J-STAGE: March 22, 2009
    JOURNAL FREE ACCESS
    To know the beginning age of the anticline growth near the Omagari Fault in Neogene siliceous rocks, northern Hokkaido, Japan, this study is focused on determining differences in sedimentation rate as the anticline grew during sedimentation, at the sites of two boreholes, which are proximal (HDB-11) and distal (HDB-10), respectively, with respect to the anticlinal axis. We have investigated i) the petrographic and chronologic characteristics of tuff layers within mudstones in terms of several parameters: modal compositions, glass refractive indexes, glass chemical compositions and fission track ages; ii) the depth distribution of the opal-A/opal-CT diagenetic boundary between diatomaceous mudstone and siliceous mudstone; and iii) the effective porosity of the mudstones. Two tuff layers as key beds, HT1 and HT2, were found at HDB-10 and HDB-11, and yielded fission track ages of 2.9 Ma and 2.2 Ma, respectively. The sedimentation rates at HDB-10 and HDB-11 between HT1 and HT2 were estimated to match with the inferred basin trend or to have been nearly equal, based on their fission track ages, the thicknesses and the effective porosity of the mudstones. The depth distribution of the opal-A/opal-CT boundary and the effective porosity indicated that the maximum burial depth of HT2 was significantly shallower at HDB-11 than at HDB-10. Previous studies have found that maximum burial occurred at/before 1.0 Ma, suggesting that the sedimentation rate at HDB-11 was slower than at HDB-10 for the 2.2∼1.0 Ma period overall. We therefore infer that the anticline began to grow between 2.2 and 1.0 Ma. The inception of the anticline growth might be caused by the beginning of the active tectonism in and around this area.
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  • Michio Tagiri, Akiko Aoi, Katsumi Kasai, Kazuo Amano
    2008 Volume 114 Issue 6 Pages 300-313
    Published: June 15, 2008
    Released on J-STAGE: March 22, 2009
    JOURNAL FREE ACCESS
    Chemical composition and K-Ar age of Miocene volcanic rocks were determined in the Daigo district, Ibaraki Prefecture. K-Ar ages of biotite for the volcanism range from 18.0 to 15.5 Ma. Compiling stratigraphy and isotopic ages revealed three magma types between N6 and N8 of planktonic foraminineferal biostratigraphy in the Daigo district. Early and middle volcanisms in the district were active as calc-alkali magma and island arc tholeiitic magma. Late volcanism is of oceanic tholeiitic magma.
    We compared the chemical characteristics of volcanism and the stratigraphic succession in the Daigo district with that found in the Motegi district of Tochigi Prefecture based on a review of the literature. CHRON stratigraphy placed the Early Miocene formations of the Motegi district between C5En and C5Br which corresponds to the zone between N6 and N8 of planktonic foramineferal biostratigraphy. In addition, isotopic age analysis of the volcanism revealed that these formations range in age from 18.6 to 15.9 Ma, indicating that the Motegi Formation is thus comparable with the early Miocene formations of the Daigo district. Moreover, icelandites from the Motegi Formation are comparable with icelandites of the Nantaisan Volcanic Breccia in the Daigo district, and stratigraphy revealed that twice oceanic-type volcanisms occurred from N6 to N8 in these districts. Twice oceanic-type volcanisms took place in the opening-stage of the Japan Sea. The former is related with the rifting, and the latter with the strike-slip fault during the opening of the Japan Sea.
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Short Article
  • Takayuki Uchino, Makoto Kawamura, Chitaro Gouzu, Hironobu Hyodo
    2008 Volume 114 Issue 6 Pages 314-317
    Published: June 15, 2008
    Released on J-STAGE: March 22, 2009
    JOURNAL FREE ACCESS
    Conglomerate containing garnet-bearing pelitic schist pebble was obtained from the Nedamo Terrane, an Early Carboniferous accretionary prism of northeast Japan. Phengite in the schist pebble is dated to be 347-317 Ma based on the 40Ar/39Ar spot-heating method. The schist in the pebble can be correlated with the Renge Metamorphic Rocks of southwest Japan, and the Yamagami Metamorphic Rocks in the Kitakami-Abukuma massif. The age data suggest that high-P/T metamorphism which produced the garnet-bearing pelitic schist may have been in progress within Early Carboniferous (359-318 Ma) and the schists were rapidly exhumed to provide pebbles, or that the conglomerate containing the pebbles may belong to a post-Early Carboniferous accretionary unit which is not identified yet. In case that the age has been all rejuvenated by secondary effects, the schist pebble may have been derived from pre-Late Devonian high-P/T metamorphics as the Matsugadaira Metamorphic Rocks, probably 380 Ma.
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